There's an odd moment in Ian McEwan's new novel, when the narrator, Joe Rose, is being interviewed by the police after a murder attempt in a restaurant. Asked what flavour of ice cream he was eating before the shooting, he replies: 'Apple'. It's not simply that this goes against the testimony of other witnesses, who remember the attack occurring fractionally earlier, the sorbets tainted with blood before they could reach the lunchers who had ordered them, but it contradicts the version we were given earlier, minimally detailed but easily remembered 10 pages later: 'The flavour of my sorbet was lime, just to the green side of white'.

Immediately before he lies to the police, or to himself, or merely the reader, Joe has been thinking about a truth free of self-interest, doubting whether a willed objectivity can save us from our engrained habits of mind, and has even asked explicitly, in a sentence standing alone as a paragraph: 'But exactly what interests of mine were served by my own account of the restaurant lunch?'

McEwan is anything but a crude writer, even when he chooses extreme subject matter, and such a sharp-elbowed nudge to the reader is out of character. To introduce at this late stage an unreliable narrator is perverse: it recapitulates on the level of gimmick, the novel's central theme, that unreliability is an ineradicable part of what we are.

Enduring Love starts with a set-piece, a ballooning accident whose most agonising aspect is that five men - Joe being one of them - are for a moment hanging by ropes from the wind-buffeted basket. If they all hold on, their combined weight will keep the basket's occupant, a 10-year-old boy, safely aground. But when one of them drops off, it rapidly becomes a race not to be the last one holding on, the one who will not have time to jump before the ascent of the balloon makes escape impossible.

It speaks well for Ian McEwan's descriptive powers and the fluency of his invention that this opening scene doesn't smell like essence of quandary, a carefully contrived human theorem, although his choice of profession for Joe a popularising science writer makes his mouthpiece almost too exquisitely adept at analysing its implications: 'This is our mammalian conflict what to give to others, and what to keep for yourself.' This, though, is the novel's painful point, as shown by the repercussions of the tragedy, that knowing more about the factors that determine your behaviour is not the same thing as becoming either freer or wiser.

In the immediate aftermath of the accident, Joe has a brief exchange with Jed Parry, one of the other helpers who was unable to keep the faith and hang on to the interests of the group. Jed subsequently develops an obsessive interest in Joe, an interest that is partly religious, partly sexual and wholly crazed. He waits outside Joe's flat in a state of transcendental infatuation, though he hides when Joe's girlfriend Clarissa comes into view.

Rationality is a precious and precarious construct in the novel, not an instinct but an achievement, a sandcastle no sooner built than washed away by the tides of the mind. The woman widowed in the accident turns out, when Joe contacts her (to exorcise his sense of guilt), to have an obsession of her own. She is convinced that her husband, a notably cautious man, must have been trying to impress someone and therefore, inevitably, a lover unseen by the others when he held on to the rope too fatally long.

Joe reacts reasonably, as he sees it, to Parry's loving persecution, but doesn't take Clarissa along with him. She begins to suspect Joe of collusion with Parry, and then of inventing Parry's obsession for reasons of his own. It's true that Parry sends ecstatic letters, but the handwriting is suspiciously similar to Joe's own.

The collapse of a couple under pressure is a recurrent McEwan theme, though he steers clear of the adulterous clich&eacutes. In The Comfort of Strangers the couple was vulnerable to destruction by reason of a sort of cosiness, an assumption of safety. In The Child In Time, it was unshareable grief that drove two people apart when they most needed each other. In The Innocent, an idyllic romance couldn't survive the impact of horror, although neither party was to blame for its eruption into their lives.

The couple is the smallest possible viable society; the breakdown between Joe and Clarissa is the subtlest variation yet on the theme. A lovingly maintained fabric that seemed to have no dangling threads unravels thoroughly.

This relationship is part of what is referred to in the title, but there is also 'enduring', in the sense of being on the receiving end of, as Joe is of Parry's mystical love. Joe makes sense of Parry's infatuation by classifying it as an instance of a pathological condition, 'de Cl&eacuterambault's syndrome', one of whose peculiarities is, ironically, that it can last indefinitely, since it isn't dependent on reciprocation. 'Enduring love', with a vengeance. Joe wants to see this syndrome as 'a dark, distorting mirror that reflected and parodied a brighter world of lovers whose reckless abandon to their cause is sane', but his own experience calls into question any so confident a separation of healthy from diseased.

Previous McEwan novels have contained genre elements: The Cement Garden, The Comfort Of Strangers and The Innocent all shared a boundary with the horror story, and a few moments in the new novel demonstrate that he is not above raising the occasional goosebump ('six months later I came across a chip of bone under the sofa'). But a story that begins with a set piece builds to no comparable climax.

Obsessed young Jed Parry, with his ponytail and the sudden inheritance that gives him the leisure to collapse emotionally, is like a Ruth Rendell character, except that in a Ruth Rendell novel he would have more access to the point of view. It's disappointing that a book that begins so full-throatedly should end with stagy confrontation, then case history, references and appendices.

At one time, it would have seemed inconceivable for Ian McEwan to write a novel with a childless couple at its heart, so central did parenthood seem to his idea of human completeness. Clarissa is unable to conceive, and has adjusted to this condition with grace and warmth, by involving herself strongly with her many godchildren. Still, from time to time, 'the unconceived child' briefly stirs in her. The theme of parenting re-emerges near the end of the book, but Clarissa for all her grounded emotions and insights, has a lower status than, say, Julie in The Child In Time. McEwan's emotional engagement with feminism is less deferent than once it was.

Joe is a jack of all sciences, while Clarissa is an academic whose speciality is Keats. McEwan can't resist equipping Joe with a full expressive panoply of language. In theory, he and she occupy different worlds, in practice he inhabits both - one chapter is even done from his imagining of her point of view, with Joe presented in the third person. Yet this imbalance is compensated for by the complexity of Joe's viewpoint, which embodies McEwan's fascination with science.

Joe reveres the hard science which once seemed his destined career, but can no longer aspire to it. His attempts to analyse behaviour without distortion are always being undone by needs he can't acknowledge, and he's at his most romantic when his language claims a scientific objectivity. Feeling a lurch of surprised love whenever he sees Clarissa after an absence, he tries to reconcile a unique pang with the big picture: 'Perhaps such amnesia is functional those who could not wrench their hearts and minds from their loved ones were doomed to fail in life's struggles and leave no genetic footprints.'

McEwan's last novel, Black Dogs, was oddly schematic, a lifeless conflict between reductive and open ways of looking at the world. Enduring Love is much the better book, despite its inability fully to dramatise its themes, perhaps because McEwan himself is richly divided between Joe's rationalism and something else.

When Joe rails against the poor science holdings of the London Library, and the assumption that the world is best understood through humanist culture, he forgets that his own livelihood as a populariser depends on there being a gap for him to bridge. So too the future of fiction is assured as long as direct self-knowledge is unattainable, and human reliability is always the good news for novelists, as well as the bad.